An adjustable valve mechanism is often applied to an internal combustion engine mounted on a vehicle for the enhanced output, the reduced fuel consumption, and the reduced emission. The adjustable valve mechanism varies a valve characteristic, such as an open-close timing or an opening-closing lift quantity of a valve.
Diverse techniques have been proposed to diagnose an abnormality of this adjustable valve mechanism. For example, the technique disclosed in Japanese Patent Laid-Open Gazettes No. 2002-161789 and No. 2001-303999 given below calculates a difference between a target phase angle of a cam shaft and an actual phase angle measured by a cam angle sensor and compares the calculated difference with a preset threshold value to determine abnormality or normality of the adjustable valve mechanism. The technique disclosed in Japanese Patent Laid-Open Gazette No. 10-18869 given below detects a phase difference between a crankshaft and the cam shaft corresponding to each control state of the adjustable valve mechanism and compares the detected phase difference with a predetermined reference value for the control state, so as to diagnose an abnormality of the adjustable valve mechanism.
The technique disclosed in Japanese Patent Laid-Open Gazettes No. 2002-161789 and No. 2001-303999 determines abnormality of the adjustable valve mechanism, based on a result of the comparison between the target phase angle and the actually observed phase angle. For example, when the target phase angle is set at a higher speed than the response of the adjustable valve mechanism, a phase difference inevitably arises even in the normal state of the adjustable valve mechanism. Namely the abnormality of the adjustable valve mechanism is mistakenly detected. The abnormality diagnosis technique having the higher accuracy has thus been highly demanded.
The technique disclosed in Japanese Patent Laid-Open Gazette No. 10-18869 assumes two control states, that is, a state of effective control and a state of no control, as the control state of the adjustable valve mechanism. The latest adjustable valve mechanism, however, can continuously vary the open-close timing or the opening-closing lift quantity of the valve. Complicated situation-based settings of the reference value are thus required corresponding to the continuously varying open-close timing or opening-closing lift quantity of the valve. An extremely large amount of labor is then required for experiments to validate the settings of the reference value. It is accordingly difficult to apply the reference values to another type of the vehicle. Another related document is Japanese Patent Laid-Open Gazette No. 2000-110594.